U.S. patent number 10,822,228 [Application Number 16/790,531] was granted by the patent office on 2020-11-03 for process for forming inkjet nozzle devices.
This patent grant is currently assigned to Memjet Technology Limited. The grantee listed for this patent is Memjet Technology Limited. Invention is credited to Gregory McAvoy, Angus North, Ronan O'Reilly.
![](/patent/grant/10822228/US10822228-20201103-D00000.png)
![](/patent/grant/10822228/US10822228-20201103-D00001.png)
![](/patent/grant/10822228/US10822228-20201103-D00002.png)
![](/patent/grant/10822228/US10822228-20201103-D00003.png)
![](/patent/grant/10822228/US10822228-20201103-D00004.png)
![](/patent/grant/10822228/US10822228-20201103-D00005.png)
![](/patent/grant/10822228/US10822228-20201103-D00006.png)
![](/patent/grant/10822228/US10822228-20201103-D00007.png)
![](/patent/grant/10822228/US10822228-20201103-D00008.png)
![](/patent/grant/10822228/US10822228-20201103-D00009.png)
![](/patent/grant/10822228/US10822228-20201103-D00010.png)
View All Diagrams
United States Patent |
10,822,228 |
North , et al. |
November 3, 2020 |
Process for forming inkjet nozzle devices
Abstract
A process for forming inkjet nozzle devices on a frontside
surface of a wafer substrate. The process includes the steps of:
(i) providing the wafer substrate having a plurality of etched
holes defined in the frontside surface, each etched hole being
filled with first and second polymers such that the second polymer
is coplanar with the frontside surface; (ii) forming the inkjet
nozzle devices on the frontside surface using MEMS fabrication
steps; and (iii) removing the first and second polymers via
oxidative ashing, wherein first and second polymers are
different.
Inventors: |
North; Angus (Sydney,
AU), O'Reilly; Ronan (Dublin, IE), McAvoy;
Gregory (Dublin, IE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Memjet Technology Limited |
Dublin |
N/A |
IE |
|
|
Assignee: |
Memjet Technology Limited
(IE)
|
Family
ID: |
1000005155641 |
Appl.
No.: |
16/790,531 |
Filed: |
February 13, 2020 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200180949 A1 |
Jun 11, 2020 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
16409687 |
May 10, 2019 |
10597290 |
|
|
|
15623267 |
Jun 25, 2019 |
10329146 |
|
|
|
15046239 |
Jul 18, 2017 |
9708183 |
|
|
|
62117385 |
Feb 17, 2015 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B41J
2/1631 (20130101); B41J 2/1628 (20130101); B41J
2/1645 (20130101); B41J 2/1603 (20130101); B81C
1/00611 (20130101); B41J 2/1639 (20130101); B81C
2201/0104 (20130101); B81B 2203/0353 (20130101); B81B
2201/052 (20130101); B81C 2201/0121 (20130101) |
Current International
Class: |
B81C
1/00 (20060101); B41J 2/16 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Olsen; Allan W.
Attorney, Agent or Firm: Cooley LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of application Ser. No.
16/409,687, entitled PROCESS FOR FILLING ETCHED HOLES USING FIRST
AND SECOND POLYMERS, filed May 10, 2019, which is a continuation of
application Ser. No. 15/623,267, entitled PROCESS FOR FILLING
ETCHED HOLES USING PHOTOIMAGEABLE THERMOPLASTIC POLYMER, filed Jun.
14, 2017, which issued as U.S. Pat. No. 10,329,146 on Jun. 25,
2019, which is a continuation of application Ser. No. 15/046,239,
entitled PROCESS FOR FILLING ETCHED HOLES, filed on Feb. 17, 2016,
which issued as U.S. Pat. No. 9,708,183 on Jul. 18, 2017, which
claims priority under 35 U.S.C. .sctn. 119(e) to U.S. Provisional
Patent Application Ser. No. 62/117,385, entitled PROCESS FOR
FILLING ETCHED HOLES, filed on Feb. 17, 2015, the content of each
of which is incorporated by reference herein in its entirety for
all purposes.
Claims
The invention clamied is:
1. A process for forming inkjet nozzle devices on a frontside
surface of a wafer substrate, the process comprising the steps of:
(i) providing the wafer substrate having a plurality of etched
holes defined in the frontside surface, each etched hole being
filled with first and second polymers such that the second polymer
is coplanar with the frontside surface; (ii) forming the inkjet
nozzle devices on the frontside surface using one or more MEMS
fabrication steps; (iii) removing the first and second polymers via
oxidative ashing, wherein the first and second polymers are
different.
2. The process of claim 1, wherein each hole has a depth of at
least 10 microns.
3. The process of claim 1, wherein each hole has an aspect ratio of
>1:1.
4. The process of claim 1, wherein the first polymer is less
viscous than the second polymer.
5. The process of claim 1, wherein the first polymer is a
thermoplastic polymer.
6. The process of claim 1, wherein the second polymer is
photoimageable.
7. The process of claim 1, wherein second polymer is superjacent
the first polymer.
8. The process of claim 1, wherein each inkjet nozzle device
comprises a nozzle chamber in fluid communication with at least one
hole.
9. The process of claim 8, wherein a respective inlet for each
nozzle chamber is defined by one of said holes.
10. The process of claim 1, further comprising the steps of: wafer
thinning and backside etching of ink supply channels.
11. The process of claim 10, wherein each ink supply channel meets
with one or more filled holes.
12. The process of claim 11, wherein each ink supply channel is
relatively wider than said one or more holes.
Description
FIELD OF THE INVENTION
This invention relates to a process for filling etched holes. It
has been developed primarily to improve the planarity of filled
holes in order to facilitate subsequent MEMS fabrication steps.
BACKGROUND OF THE INVENTION
The Applicant has developed a range of Memjet.RTM. inkjet printers
as described in, for example, WO2011/143700, WO2011/143699 and
WO2009/089567, the contents of which are herein incorporated by
reference. Memjet.RTM. printers employ a stationary pagewidth
printhead in combination with a feed mechanism which feeds print
media past the printhead in a single pass. Memjet.RTM. printers
therefore provide much higher printing speeds than conventional
scanning inkjet printers.
In order to minimize the amount of silicon, and therefore the cost
of pagewidth printheads, each Memjet.RTM. printhead IC is
fabricated via an integrated CMOS/MEMS process to provide a high
nozzle packing density. A typical Memjet.RTM. printhead IC contains
6,400 nozzle devices, which translates to 70,400 nozzle devices in
an A4 printhead containing 11 Memjet.RTM.printhead ICs.
As described in U.S. Pat. No. 7,246,886, the contents of which are
incorporated herein by reference, a typical printhead fabrication
process for Memjet.RTM. printhead ICs requires etching of holes in
a frontside of a CMOS wafer via DRIE (deep reactive ion etching),
filling the holes with a sacrificial material (e.g. photoresist) to
provide a planar frontside surface, and then subsequently building
MEMS nozzle devices on the frontside of the wafer. After completion
of the all frontside MEMS fabrication steps, the wafer is thinned
from the backside and trenches are etched from the backside to meet
with the filled frontside holes. Finally, all sacrificial material
is removed from frontside holes and MEMS nozzle chambers by
oxidative ashing. In the resulting printhead IC, the frontside
holes define individual inlet channels for nozzle chambers.
A critical stage of fabrication is plugging the frontside holes
with sacrificial material and planarizing the frontside surface of
the wafer. If the frontside surface is not fully planar, then any
lack of planarity is carried through subsequent MEMS fabrication
steps and, ultimately, may lead to defective devices or weakened
MEMS structures with shorter installed lifetimes.
One process for plugging holes formed by DRIE is described in U.S.
Pat. No. 7,923,379. In this prior art process, a hole is filled in
multiple stages by spinning on sequential layers of a photoresist.
After each of these stages, the photoresist on the front surface of
the wafer is selectively exposed and developed to leave only
photoresist partially filling the hole. The remaining photoresist
inside the hole is hardbaked and the process repeated until the
hole is fully filled with photoresist. The aim is to provide a hole
plugged with photoresist at the end of the process, whereby an
upper surface of the photoresist plug is coplanar with a frontside
surface of wafer. This is the ideal foundation for subsequent MEMS
fabrication steps on the frontside surface of the wafer.
However, the process described in U.S. Pat. No. 7,923,379 has a
number of drawbacks. Firstly, it is not possible to achieve true
planarity at the end of the process, because the hole is usually
slightly overfilled or underfilled after the final exposure and
development steps. Secondly, photoresist is highly viscous, which
inhibits the escape of solvent or air bubbles. Bubbles can escape
from the relatively thin final layer of photoresist, but cannot
readily escape from the layer(s) of photoresist at the bottom of
the hole. During thermal curing, these trapped solvent bubbles may
combine and expand to form relatively large voids, with consequent
instability in the plug. Thirdly, photoresists typically contract
during thermal curing (`hardbaking`). Contraction of the
photoresist during hardbaking also affects the stability of the
plug. Thus, even if a planar upper surface can be achieved, the
photoresist plug may be susceptible to `dishing` during subsequent
MEMS fabrications steps; and any lack of stability in the
photoresist plug may lead to problems in subsequently constructed
MEMS structures e.g. nozzle plate cracking.
Thermoplastic polymers, which typically have lower viscosities than
most photoresists and can be reflowed when heated, offer a
potential solution to at least some of the problems associated with
trapped solvent bubbles and contraction of photoresist as described
above. However, thermoplastic polymers are not usually
photoimageable and require planarizing via a chemical-mechanical
planarization (CMP) process. Although a CMP process is technically
possible for thermoplastic polymers, it is not practically feasible
for thick layers of polymer, which are required to fill relatively
deep holes formed by DRIE. This is due to: (1) poor stopping
selectivity on the frontside surface when planarizing thick layers
of polymer; (2) the rate of CMP being unacceptably slow for large
scale fabrication; (3) rapid `gumming` of CMP polishing pads, which
consequently require regular replacement.
It would be desirable to provide an alternative process for filling
photoresist holes, which ameliorates at least some of the problems
described above.
SUMMARY OF THE INVENTION
In a first aspect, there is provided a process for filling one or
more etched holes defined in a frontside surface of a wafer
substrate, said process comprising the steps of:
(i) depositing a layer of a thermoplastic first polymer onto the
frontside surface and into each hole;
(ii) reflowing the first polymer;
(iii) exposing the wafer substrate to a controlled oxidative plasma
so as to reveal the frontside surface;
(iv) optionally repeating steps (i) to (iii);
(v) depositing a layer of a photoimageable second polymer so as to
overfill each hole with said second polymer;
(vi) selectively removing the second polymer from regions outside a
periphery of the holes to provide overfilled holes, the selective
removing comprising exposure and development of the second polymer;
and
(vii) planarizing the frontside surface to provide one or more
holes filled with a plug comprising the first and second polymers,
each plug having a respective upper surface coplanar with the
frontside surface,
wherein the first and second polymers are different.
The process according to the first aspect advantageously provides a
robust process for plugging high aspect ratio holes formed by DRIE.
In particular, the process provides a plug which is substantially
free of bubbles by virtue of using a relatively low viscosity first
polymer having thermoplastic reflow properties, which allows
bubbles to readily escape during deposition and reflow. Further,
the process provides a stable foundation for subsequent MEMS
processes by virtue of employing a reflowable thermoplastic first
polymer, which uniformly fills the frontside hole. Still further,
the process provides a frontside plug having an upper surface
coplanar with the frontside surface by virtue of planarizing step
(typically chemical-mechanical planarizing). Planarization (e.g. by
CMP) is facilitated by use of the photoimageable second polymer for
the final filling step, which is removed from regions outside the
periphery of each hole by conventional exposure and development.
Thus, a minimal amount of the second polymer needs to be removed by
planarization, which enables high throughput, good stopping
selectivity and minimal gumming of CMP polishing pads (i.e. lower
consumable costs). These and other advantages will be apparent to
the person skilled in the art from the detailed description of the
first embodiment below.
Preferably, the first polymer is less viscous than the second
polymer. As foreshadowed above, a relatively low viscosity first
polymer facilitates escape of trapped solvent and air bubbles,
resulting in a more robust plug.
Preferably, each hole has a depth of at least 5 microns or at least
10 microns. Typically, each hole has depth in the range of 5 to 100
microns or 10 to 50 microns.
Preferably, each hole has an aspect ratio of >1:1. Typically,
the aspect ratio is in the range of 1.5-5:1
In one embodiment, steps (i) to (iii) may be repeated one or more
times. In other embodiments, steps (i) to (iii) may be performed
only once. In an alternative embodiment, steps (i) and (ii) may be
repeated one or more times, and step (iii) may be performed only
once.
Preferably, an extent of overfill of the hole immediately prior to
step (vi) is less than about 12 microns or less than about 10
microns. Minimal overfill is desirable to facilitate subsequent
planarization.
Typically, additional MEMS fabrication steps are performed on the
planarized frontside surface of the wafer substrate. In a preferred
embodiment, the additional MEMS fabrication steps construct inkjet
nozzle devices on the planarized frontside surface of the wafer
substrate. Each nozzle device may comprise a nozzle chamber in
fluid communication with at least one hole, and a respective inlet
for each nozzle chamber may be defined by one of said holes.
Preferably, the additional MEMS fabrication steps include at least
one of: wafer thinning and backside etching of ink supply channels.
Each ink supply channel preferably meets with one or more filled
holes to provide fluid connections between the backside and
frontside of the wafer. Each ink supply channel is usually
relatively wider than the frontside holes.
A final stage of MEMS fabrication preferably employs oxidative
removal ("ashing") of the first and second polymers from the holes.
Oxidative removal typically employs an oxygen-based plasma, as
known in the art.
In a second aspect, there is provided a process for filling one or
more etched holes defined in a frontside surface of a wafer
substrate, said process comprising the steps of:
(i) depositing a layer of a photoimageable thermoplastic third
polymer onto the frontside surface and into each hole;
(ii) reflowing the third polymer;
(iii) selectively removing the third polymer from regions outside a
periphery of each hole, the selective removing comprising exposure
and development of the third polymer;
(iv) optionally repeating steps (i) to (iii) until each hole is
overfilled with the third polymer; and
(v) planarizing the frontside surface to provide one or more holes
filled with a plug of the third polymer, each plug having a
respective upper surface coplanar with the frontside surface.
The process according to the second aspect makes use of a special
class of thermoplastic photoimageable polymers. The desirable
property of thermoplasticity enables the third polymer to be
reflowed so as to enjoy the same advantages as those described
above in connection with the first polymer. Furthermore, the
desirable property of photoimageability enables the third polymer
to be removed from regions outside a periphery of the holes by
conventional photolithographic exposure and development.
Accordingly, the process according to the second aspect obviates
oxidative removal of the first polymer (as described above in
connection with the first aspect), whilst still enjoying the
advantages of: a highly robust plug; coplanarity of the plug and
frontside surface following planarizing; and efficient
planarization by virtue of photolithographic removal of the
majority of the third polymer prior to planarization.
Preferably, the process according to the second aspect comprises
only a single sequence of steps (i) to (iii), wherein each hole is
overfilled with the third polymer after step (iii).
Other preferred embodiments, where relevant, which are described
above in connection with the first aspect are of course applicable
to the second aspect.
In a third aspect, there is provided a process for filling one or
more etched holes defined in a frontside surface of a wafer
substrate, the process comprising the steps of:
(i) depositing a layer of a thermoplastic first polymer onto the
frontside surface and into each hole;
(ii) reflowing the first polymer;
(iii) optionally repeating steps (i) and (ii) until the holes are
overfilled with the first polymer;
(iv) depositing a layer of a photoimageable second polymer;
(vi) selectively removing the second polymer from regions outside a
periphery of the holes, the selective removing comprising exposure
and development of the second polymer;
(vii) exposing the wafer substrate to a controlled oxidative plasma
so as to reveal the frontside surface of the wafer substrate;
and
(viii) planarizing the frontside surface to provide one or more
holes filled with a plug comprising the first polymer only, each
plug having a respective upper surface coplanar with the frontside
surface,
wherein the first and second polymers are different.
The process according to the third aspect is analogous in many
respects to the process according to the first aspect. However, in
the third aspect, the second polymer is used merely to provide a
relatively thicker polymeric layer over each hole, each hole being
initially overfilled with the first polymer. Therefore, the
oxidative removal step ensures that a cap of polymeric material
remains over each hole prior to planarization. This is advantageous
because any solvent or air bubbles in the second polymer, which may
be present at the interface between the first and second polymers,
are removed during the planarization step. Hence, the plug of
material filling the hole is solely the thermoplastic first
polymer, which provides a very robust foundation for subsequent
MEMS fabrication steps.
In some embodiments, the process may comprise the additional step
of: exposing the wafer substrate to a controlled oxidative plasma
so as to reveal the frontside surface of the wafer substrate after
step (ii).
Other preferred embodiments, where relevant, which are described
above in connection with the first aspect are of course applicable
to the third aspect.
In a fourth aspect, there is provided a process for filling one or
more etched holes defined in a frontside surface of a wafer
substrate, said process comprising the steps of:
(i) depositing a layer of a photoimageable fourth polymer onto the
frontside surface and into each hole;
(ii) selectively removing the fourth polymer from regions outside a
periphery of each hole, the selective removing comprising exposure
and development of the fourth polymer;
(v) optionally repeating steps (i) and (ii) until each hole is
overfilled with the fourth polymer; and
(vi) planarizing the frontside surface to provide one or more holes
filled with a plug of the fourth polymer, each plug having a
respective upper surface coplanar with the frontside surface.
The process according to the fourth aspect is most suitable for
filling relatively shallower (i.e. less than 10 microns) or low
aspect ratio (i.e. less than 1:1) holes. The fourth polymer is
typically conventional photoresist, which is not thermoplastic and
cannot, therefore, be reflowed. Nevertheless, efficient
planarization is still achievable since the amount of fourth
polymer to be removed by CMP is minimized.
Other preferred embodiments, where relevant, which are described
above in connection with the first aspect are of course applicable
to the third aspect.
As used herein, the term "hole" generally means any cavity, via or
trench defined in a wafer substrate. By definition, each hole has a
floor and sidewalls extending upwards therefrom to meet with a
surface of the wafer substrate. Each hole may have any shape in
cross-section, such as circular, oblong, rounded oblong, square,
rounded square, oval, elliptical etc. Likewise, the hole may be in
the form of an elongate trench. In the present context, elongate
trenches may be used as `dicing streets` for dicing silicon wafers
into individual chips.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described by way
of example only with reference to the accompanying drawings, in
which:
FIG. 1 is a schematic side view of a silicon substrate having a
high aspect ratio hole etched in frontside surface;
FIG. 2 shows the substrate shown in FIG. 1 after deposition of a
thermoplastic first polymer;
FIG. 3 shows the substrate shown in FIG. 2 after reflowing and
curing of the first polymer;
FIG. 4 shows the substrate shown in FIG. 3 after oxidative removal
of the first polymer from the frontside surface;
FIG. 5 shows the substrate shown in FIG. 4 after deposition of a
photoimageable second polymer;
FIG. 6 shows the substrate shown in FIG. 5 after exposure and
development of the second polymer;
FIG. 7 shows the substrate shown in FIG. 6 after
chemical-mechanical planarization;
FIG. 8 shows the substrate shown in FIG. 1 after deposition of a
thermoplastic photoimageable third polymer;
FIG. 9 shows the substrate shown in FIG. 8 after reflowing and
curing of the third polymer;
FIG. 10 shows the substrate shown in FIG. 9 after exposure and
development of the third polymer;
FIG. 11 shows the substrate shown in FIG. 10 after
chemical-mechanical planarization;
FIG. 12 shows the substrate shown in FIG. 1 after repeated
deposition and reflow baking of the thermoplastic first
polymer;
FIG. 13 shows the substrate shown in FIG. 12 after deposition of
the photoimageable second polymer;
FIG. 14 shows the substrate shown in FIG. 13 after exposure and
development of the second polymer;
FIG. 15 shows the substrate shown in FIG. 14 after oxidative
removal of the first polymer from the frontside surface;
FIG. 16 shows the substrate shown in FIG. 15 after
chemical-mechanical planarization;
FIG. 17 is a schematic side view of a silicon substrate having a
low aspect ratio hole etched in frontside surface;
FIG. 18 shows the substrate shown in FIG. 17 after deposition of a
conventional photoimageable polymer;
FIG. 19 shows the substrate shown in FIG. 18 after exposure and
development;
FIG. 20 shows the substrate shown in FIG. 10 after
chemical-mechanical planarization;
FIG. 21 is a perspective view of inkjet nozzle devices each having
a chamber inlet defined in a frontside surface of a silicon
substrate; and
FIG. 22 is a sectional side view of the inkjet nozzle device shown
in FIG. 21.
DETAILED DESCRIPTION OF THE INVENTION
First Embodiment
Referring to FIG. 1, there is shown a substrate 1 having a high
aspect ratio hole 2 defined in a frontside surface 3 thereof. The
substrate is a CMOS silicon wafer having an upper CMOS layer 5
disposed on a bulk silicon substrate 4. The CMOS layer 4 typically
comprises one more metal layers interposed between interlayer
dielectric (ILD) layers. The hole 2 may be defined by any suitable
anisotropic DRIE process (e.g. `Bosch etch` as described in U.S.
Pat. No. 5,501,893). The hole 2 may have any desired shape in
cross-section, the shape being defined by a photoresist mask during
the etching process.
FIG. 2 shows the substrate 1 after spin-coating a reflowable
thermoplastic polymer 7 onto the frontside surface 3 followed by
soft-baking. The thermoplastic polymer 7 is non-photoimageable and
may be of any suitable type known to those skilled in the art. For
example, the thermoplastic polymer 7 may be an adhesive, such as a
polyimide adhesive. A specific example of a suitable thermoplastic
polymer 7 is HD-3007 Adhesive, available from HD
MicroSystems.TM..
Soft-baking after deposition of the thermoplastic polymer 7 removes
solvent to provide a tack-free film. Since the thermoplastic
polymer 7 has a relatively low viscosity (e.g. <1500 Cps), any
air or solvent bubbles present in the polymer can readily escape
during soft-baking. Still referring to FIG. 2, it can be seen that
the thermoplastic polymer 7 is readily deposited inside the high
aspect ratio hole 2 during spin-coating due to it relatively low
viscosity.
Referring now to FIG. 3, there is shown the substrate 1 after
reflow-baking at a relatively higher temperature than soft-baking.
This reflow-baking step raises the thermoplastic polymer 7 to a
temperature above its glass transition temperature, allowing the
polymer to reflow and fill the hole 2 more completely. For example,
reflow-baking may be performed at about 300.degree. C., while
soft-baking may be performed at about 90.degree. C.
Depending on the depth and aspect ratio of the hole 2, as well as
the type of thermoplastic polymer 7 employed, the steps described
in connection with FIGS. 2 and 3 may be repeated one or more times
until the hole is filled to a level just below the frontside
surface, as shown in FIG. 3. The hole 2 may be >60% filled,
>70% filled, >80% or >90% after all spin-coating and
reflowing steps have been completed.
After the hole 2 has been partially-filled to a desired level, the
thermoplastic polymer 7 is then cured at a relatively higher
temperature than the reflow baking temperature in order to
cross-link and harden the polymer. The resultant plug of
thermoplastic polymer 7 shown in FIG. 3 is substantially free of
any air or solvent bubbles. Moreover, the reflow step(s) ensure the
thermoplastic polymer 7 uniformly contacts sidewalls of the hole 2
to provide a robust foundation for subsequent MEMS processing.
Turning now to FIG. 4, the substrate 1 is shown after removal of a
predetermined thickness of the thermoplastic polymer 7 via a
controlled oxidative removal process ("ashing"). Typically, the
controlled oxidative removal process comprises a timed exposure to
an oxygen-based plasma in a conventional ashing oven. A planar
thickness of polymer removed by the ashing process is proportional
to the period of ashing. As shown in FIG. 4, the ashing process
removes a thickness of the thermoplastic polymer 7, such that
removal is complete from the frontside surface 3 in regions outside
the periphery of the hole 2. However, the hole 2 remains
partially-filled with the thermoplastic polymer 7 by virtue of the
additional thickness of polymer in the hole.
Next, as shown in FIG. 5, a conventional photoimageable
(non-thermoplastic) polymer 9 is deposited onto the frontside
surface 3 of the substrate 1 by spin-coating followed by
soft-baking. The photoimageable polymer 9 is spin-coated to a
thickness of about 8 microns so as to overfill the hole 2. The
photoimageable polymer 9 may be of any suitable type known to those
skilled in the art. For example, the photoimageable polymer 9 may
be a polyimide or a conventional photoresist. A specific example of
a suitable photoimageable polymer 9 is HD-8820 Aqueous Positive
Polyimide, available from HD Micro Systems.TM..
Referring to the FIG. 6, the photoimageable polymer 9 is then
exposed and developed, by conventional methods known to those
skilled in the art, so as to remove substantially all of the
polymer 9 from regions outside a periphery of the hole 2. The
resultant substrate 1 has an overfilled hole 2 having an 8 micron
"cap" of the photoimageable polymer 9.
Following final curing of the photoimageable polymer 9, the
frontside surface 3 of the substrate 1 is then subjected to
chemical-mechanical planarization (CMP) so as to remove the cap of
photoimageable polymer 9 and provide a planar frontside surface, as
shown in FIG. 7. Advantageously, the amount of photoimageable
polymer 9 that is required to be removed by CMP is relatively small
due to the previous exposure and development steps described in
connection with FIG. 6. Hence, the CMP process has acceptable
process times (e.g. 5 minutes or less), good stopping selectivity
and minimal gumming of CMP pads, which reduces the cost of
consumables.
In the resultant substrate 1, shown in FIG. 7, the hole 2 is
plugged with the thermoplastic polymer 7 and the photoimageable
polymer 9. This polymer plug is robust and substantially free of
any solvent or air bubbles. Furthermore, an upper surface 11 of the
plug is coplanar with the frontside surface 3 by virtue of the
final planarizing process. The plugged hole therefore provides an
ideal foundation for subsequent frontside MEMS processing steps,
such as fabrication of inkjet nozzle structures.
Second Embodiment
A second embodiment of the present invention will now be described
with reference to FIGS. 8 to 11. Referring firstly to FIG. 8 the
hole 2 is filled with a polymer 13 having both thermoplastic and
photoimageable properties. An example of the thermoplastic
photoimageable polymer 13 is Level.RTM. M10 coating, available from
Brewer Science. The thermoplastic photoimageable polymer 13 has a
relatively low viscosity which is comparable to the thermoplastic
polymer 7 described hereinabove. The polymer 13 is therefore able
to fill the hole 2 in a single spin-coating followed by soft-baking
to removal solvent. The low viscosity and thermoplastic reflow
properties of the polymer 13 enable any solvent or air bubbles to
escape during soft-baking and reflow baking.
FIG. 9 shows the polymer 13 after reflow-baking at a relatively
higher temperature than soft-baking. This reflow-baking step raises
the polymer 13 to a temperature above its glass transition
temperature, allowing the polymer to reflow and ensure the hole 2
is overfilled.
Referring to the FIG. 10, the thermoplastic photoimageable polymer
13 is then exposed and developed by conventional methods known to
those skilled in the art, so as to remove substantially all of the
polymer 13 from regions outside a periphery of the hole 2. The
resultant substrate 1 has an overfilled hole 2 with a "cap" of the
polymer 13.
Following final curing (e.g. UV curing) of the thermoplastic
photoimageable polymer 13, the frontside surface 3 of the substrate
1 is then subjected to chemical-mechanical planarization (CMP) so
as to remove the cap of polymer 13 and provide a planar frontside
surface, as shown in FIG. 11. Advantageously, the amount of polymer
13 that is required to be removed by CMP is relatively small due to
the previous exposure and development steps described in connection
with FIG. 10. Hence, the CMP process has acceptable process times
(e.g. 5 minutes or less), good stopping selectivity and minimal
gumming of CMP pads, which reduces the cost of consumables.
In the resultant substrate 1, shown in FIG. 11, the hole 2 is
plugged with the thermoplastic photoimageable polymer 13. This
polymer plug is robust and substantially free of any solvent or air
bubbles. Furthermore, an upper surface 15 of the plug is coplanar
with the frontside surface 3 by virtue of the final planarizing
process. The plugged hole therefore provides an ideal foundation
for subsequent frontside MEMS processing steps, such as fabrication
of inkjet nozzle structures.
Third Embodiment
Referring to FIGS. 12 to 16, there is shown a third embodiment of
the present invention employing the first polymer 7 and the second
polymer 9, as described above in connection with the first
embodiment. FIG. 12 shows the substrate 1 after spin-coating of the
thermoplastic first polymer 7 and reflow baking. By contrast with
the first embodiment, the hole 2 is overfilled with the polymer 7,
typically using two or more cycles of spin-coating and reflow
baking. After reflow baking, the substrate 1 may be exposed to an
oxidative plasma to remove the polymer 7 from the frontside surface
3. However, this step is optional and FIG. 12 shows an alternative
process where there is no ashing step after each cycle of
spin-coating and reflow baking.
Referring to FIG. 13, the photoimageable second polymer 9 is then
spin-coated on the substrate 1 over the thermoplastic polymer 7.
Subsequent masked exposure and development of the second polymer 9
removes the second polymer from regions outside a periphery of the
hole 2. Accordingly, as shown in FIG. 14, a relatively thick
polymeric layer, comprised of the first polymer and second polymer
9, is disposed over the hole 2; and a relatively thin polymeric
layer, comprised of the first polymer 7, is disposed over the
remainder of the frontside surface 3 in regions outside a periphery
of the hole 2.
Referring to FIG. 15, the substrate 1 is then exposed to a
controlled oxidative plasma ("ashing") so as to remove a
predetermined thickness of polymeric material. The first polymer 7
is removed completely from regions outside a periphery of the hole
2 to reveal the frontside surface 3. However, since a relatively
thick polymeric layer was disposed over the hole 2 prior to ashing,
a polymeric cap 17 remains over the hole after the ashing step, as
shown in FIG. 15.
Finally, as shown in FIG. 16, the frontside surface is subjected to
chemical-mechanical planarization (CMP) to remove the polymeric cap
17, stopping on the frontside surface 3. The process according to
the third embodiment advantageously provides a plug of the first
polymer 7 filling the hole 2. Moreover, an upper surface 19 of the
plug of the first polymer 7 is coplanar with the frontside surface
3.
The process according to the third embodiment is potentially
advantageous compared to the first embodiment by avoiding any of
the second polymer 9 in the final plugged hole. Therefore, any
solvent or air bubbles present in the second polymer 9, which may
grow at an interface between the first and second polymers, are
avoided in the final plugged hole.
Fourth Embodiment
The fourth embodiment described herein is suitable for filling
relatively shallow and/or low aspect ratio holes (e.g. holes having
an aspect ratio of <1:1 and/or holes have a depth of less than
10 microns or less than 5 microns). FIG. 17 shows the silicon
substrate 1 having a low aspect ratio hole 21 defined in a
frontside surface 3 thereof.
FIG. 18 shows the substrate 1 after spin-coating a conventional
photoimageable polymer 23 onto the frontside surface 3 followed by
soft-baking. The photoimageable polymer 23 may be of any suitable
type known to those skilled in the art, such as polyimide or
photoresist.
The hole 17 is intentionally overfilled with the polymer 23 and
then the polymer is subsequently removed from regions outside the
periphery of the hole by conventional exposure and development
steps. FIG. 19 shows the substrate 1 after exposure and development
of the polymer 23; the hole 17 is plugged with the polymer and has
a cap of polymeric material protruding from the frontside surface
3.
Following final curing of the photoimageable polymer 23, the
frontside surface 3 of the substrate 1 is then subjected to
chemical-mechanical planarization (CMP) so as to remove the cap of
polymer 23 and provide a planar frontside surface, as shown in FIG.
20. Advantageously, the amount of polymer 23 that is required to be
removed by CMP is relatively small due to the previous exposure and
development steps described in connection with FIG. 19. Hence, the
CMP process has acceptable process times (e.g. 5 minutes or less),
good stopping selectivity and minimal gumming of CMP pads, which
reduces the cost of consumables.
Moreover, the plug of polymer 23 has a uniform upper surface 25,
which is coplanar with the frontside surface 3. The plugged hole
therefore provides a good foundation for subsequent frontside MEMS
processing steps.
Although the process described above in connection with the fourth
embodiment employs a single hole-filling step, it will be
appreciated by those skilled in the art that the hole may be filled
in multiple stages, similar to the process described in U.S. Pat.
No. 7,923,379. However, in contrast with the process described in
U.S. Pat. No. 7,923,379, the process according to the third
embodiment overfills the hole for subsequent planarization (see
FIGS. 18 and 19).
MEMS Inkjet Nozzle Devices
By way of completeness, there will now be described an inkjet
nozzle device fabricated by leveraging the hole-filling process
described above.
Referring to FIGS. 21 and 22, there is shown an inkjet nozzle
device 10 comprising a main chamber 12 having a floor 14, a roof 16
and a perimeter wall 18 extending between the floor and the roof.
FIG. 21 shows a CMOS layer 20, which may comprise a plurality of
metal layers interspersed with interlayer dielectric (ILD)
layers.
In FIG. 21 the roof 16 is shown as a transparent layer so as to
reveal details of each nozzle device 10. Typically, the roof 16 is
comprised of a material, such as silicon dioxide or silicon
nitride.
The main chamber 12 of the nozzle device 10 comprises a firing
chamber 22 and an antechamber 24. The firing chamber 22 comprises a
nozzle aperture 26 defined in the roof 16 and an actuator in the
form of a resistive heater element 28 bonded to the floor 14. The
antechamber 24 comprises a main chamber inlet 30 ("floor inlet 30")
defined in the floor 14. The main chamber inlet 30 meets and
partially overlaps with an endwall 18B of the antechamber 24. This
arrangement optimizes the capillarity of the antechamber 24,
thereby encouraging priming and optimizing chamber refill
rates.
A baffle plate 32 partitions the main chamber 12 to define the
firing chamber 22 and the antechamber 24. The baffle plate 32
extends between the floor 14 and the roof 16.
The antechamber 24 fluidically communicates with the firing chamber
22 via a pair of firing chamber entrances 34 which flank the baffle
plate 32 on either side thereof. Each firing chamber entrance 34 is
defined by a gap extending between a respective side edge of the
baffle plate 32 and the perimeter wall 18.
The nozzle aperture 26 is elongate and takes the form of an ellipse
having a major axis aligned with a central longitudinal axis of the
heater element.
The heater element 28 is connected at each end thereof to
respective electrodes 36 exposed through the floor 14 of the main
chamber 12 by one or more vias 37. Typically, the electrodes 36 are
defined by an upper metal layer of the CMOS layer 20. The heater
element 28 may be comprised of, for example, titanium-aluminium
alloy, titanium aluminium nitride etc. In one embodiment, the
heater 28 may be coated with one or more protective layers, as
known in the art.
The vias 37 may be filled with any suitable conductive material
(e.g. copper, tungsten etc.) to provide electrical connection
between the heater element 28 and the electrodes 36. A suitable
process for forming electrode connections from the heater element
28 to the electrodes 36 is described in U.S. Pat. No. 8,453,329,
the contents of which are incorporated herein by reference.
Part of each electrode 36 may be positioned directly beneath an end
wall 18A and baffle plate 32 respectively. This arrangement
advantageously improves the overall symmetry of the device 10, as
well as minimizing the risk of the heater element 28 delaminating
from the floor 14.
As shown most clearly in FIG. 21, the main chamber 12 is defined in
a blanket layer of material 40 deposited onto the floor 14 and
etched by a suitable etching process (e.g. plasma etching, wet
etching etc.). The baffle plate 32 and the perimeter wall 18 are
defined simultaneously by this etching process, which simplifies
the overall MEMS fabrication process. Hence, the baffle plate 32
and perimeter wall 18 are comprised of the same material, which may
be any suitable etchable ceramic or polymer material suitable for
use in printheads. Typically, the material is silicon dioxide or
silicon nitride.
A printhead 100 may be comprised of a plurality of inkjet nozzle
devices 10. The partial cutaway view of the printhead 100 in FIG.
21 shows only two inkjet nozzle devices 10 for clarity. The
printhead 100 is defined by a silicon substrate 102 having the
passivated CMOS layer 20 and a MEMS layer containing the inkjet
nozzle devices 10. As shown in FIG. 21, each main chamber inlet 30
meets with an ink supply channel 104 defined in a backside of the
printhead 100. The ink supply channel 104 is generally much wider
than the main chamber inlets 30 and provides a bulk supply of ink
for hydrating each main chamber 12 in fluid communication
therewith. Each ink supply channel 104 extends parallel with one or
more rows of nozzle devices 10 disposed at a frontside of the
printhead 100. Typically, each ink supply channel 104 supplies ink
to a pair of nozzle rows (only one row shown in FIG. 21 for
clarity), in accordance with the arrangement shown in FIG. 21B of
U.S. Pat. No. 7,441,865.
The printhead 100 may be fabricated by building the MEMS layer
containing inkjet nozzle devices 10 on a wafer substrate having the
plugged hole shown in FIG. 7. The planarized frontside surface 3 of
the substrate facilitates frontside MEMS fabrication processes.
After frontside MEMS fabrication steps are completed, the wafer is
thinned from a backside and the ink supply channels 104 are etched
from the backside to meet with the plugged frontside holes.
Finally, the polymer plug (e.g. polymers 7 and 9) is removed from
the frontside hole 2 by oxidative ashing to define the main chamber
inlets 30.
It will, of course, be appreciated that the present invention has
been described by way of example only and that modifications of
detail may be made within the scope of the invention, which is
defined in the accompanying claims.
* * * * *